Lori:
First, let me say I am not a strong proponent of automatically processing messages from MRP without someone looking at them first.
That said, there is a job (R3411) to run in batch or subsystem that processes the messages from MRP based on the data selection. One client of mine ran this to release WO's only, and reviewed them before attaching parts lists and routings. They set up a specific version of the WO entry program identifying a WO status of "To be reviewed", and called that version from the R3411. They did not allow rescheduling of orders automatically. The same process could be used for PO's as well.
Good luck!